1. Field of the Invention
Embodiments disclosed herein generally relate to a downhole wiper plug system and a method of using the downhole wiper plug system. Specifically, embodiments disclosed herein relate to a system of liner wiper plugs used to isolate cement from drilling fluids when pumping cement into the formation. More specifically, embodiments disclosed herein relate to a dual wiper plug system and method of cementing a liner in a borehole using the dual wiper plug system.
2. Background Art
After a borehole has been drilled into the earth, a string of steel casing or liner is lowered and set therein. One drillable shoe and possibly one drillable collar having an upwardly closing check valve are mounted on or near the lower end of the string to prevent back flow. After the liner has been suspended by a hanger apparatus near the lower end of a previously run casing string, cement slurry is pumped down the interior thereof and out into the borehole via the check valves where it flows up in the annulus outside the liner up to a desired level. The drilling mud that was standing in the well prior to cementing is displaced and circulated out of the well during the casing setting and cementing steps. When the cement has hardened, it seals off the annular space between the outside of the liner and the surrounding well bore wall and prevents migration of formation fluids therealong.
It is desirable to protect the cement slurry from contamination by the drilling mud as the slurry is being pumped into the well. The usual practice to protect the cement slurry is to place a first plug ahead of the cement column which provides a separation between the lower end of such column and the mud, and to place a second plug which performs the same function at the top of the column. Each plug typically has a series of upwardly facing elastomer cups whose outer edges engage the inner walls of the liner to provide sliding seals and wipers. When the first plug lands against a float shoe at the bottom of the liner, a passage is opened up through the float shoe which enables cement to be pumped into the annulus. Eventually the second plug lands against the first plug as the displacement is completed. The check valves in the float shoes prevent back flow of the cement into the casing or liner during the time that it takes for the cement to set up. During downward movement, the outer edges of the cups of the second plug wipe or scrape the cement off of the inner walls of the liner so that no deposits are left. Once the cement has hardened, the plugs and cement shoes can be drilled out.
Wiper plugs used in cementing liners have been designed such that cement slurry and other fluids could be pumped through a flow passage in the plug itself, which requires complicated valve systems to open and close this passage. This complexity has resulted in plug structures that may be difficult to drill out at the end of the cementing operation. The inclusion of such valve structures also has reduced the performance characteristics of such plugs, particularly when the liner hanger and wiper plug launching system are used on directional or horizontal sections of a well.
In conventional wiper plug systems, the first and second plugs are engaged with the liner with shear screws. When a predetermined pressure is applied to the first or second plug, the shear screws break and allow the first and/or second plug to continue downward within the liner. Such shear mechanisms may be prose to prematurely releasing (i.e., breaking) if the tool is impacted when run into the hole. If the first plug is prematurely released, the plug may not properly move along the liner or properly seat in a seat of the float shoe. Moreover, if the first plug improperly impacts or lands in the seat, the seat may be damaged or debris may block the check valve.
Accordingly, there exists a need for an efficient and reliable liner wiper system.